Fix your wiring loom

Badly modified and worn wiring will make a vehicle unreliable and potentially dangerous: Alisdair Cusick explains the precautions when fitting a brand new loom

SIR William Lyons, the founder of Jaguar once said: “The car is the closest thing we will ever create to something that is alive”. Of all the jobs on a Land Rover, the single task that reflects that the most, is wiring up a vehicle. Many components go to create a vehicle, but only when they are connected electrically (or electronically) will they usually function as one. That moment when you first turn on the ignition and start the engine is always a highlight. Magically, every system suddenly comes to life.

Wiring tends to be fitted in multistrand groups of wires, which make up the loom. In its most simple terms, one side of the battery connects to the starter motor and to the ignition via the dashboard and thus to the various parts of the car as power is needed. The circuits are completed via earth leads to the vehicle chassis or body which are connected back to the battery earth terminal. The primary loom’s job is to supply the starter and ignition system, with the secondary loom supplying the various ancillaries such as lights and wipers.

Generally, up to Series IIA Land Rovers will have a positive earth system (the battery positive terminal is connected to earth), and later models have a negative earth system. I say generally, for it is common to swap early cars to negative earth, primarily for fitting an alternator instead of a dynamo, so you need to know the polarity of your vehicle. That is the only reason to take note though, for positive or negative earth, either way, you just need to remember the opposite feed to the earth is the switched feed.

Wiring up a vehicle isn’t particularly difficult, but accuracy and neatness are essential. Get it wrong, and you’ll create a dead short, where positive and negative meet. In this way the wires basically turn into toaster elements, melting insulation, with disastrous results if near fuel pipes.

Thankfully, it is possible to check for such faults using a multimeter before powering up the vehicle. A multimeter is vital for this job, not only to check continuity, for example that a wired switch correctly makes a circuit to a headlight, but also to check the circuit on both sides of any non-functioning item, such as a bulb or relay.